Direct Torque Control of a 6-Phase

Switched Reluctance Motor

Xu Deng, Prof. Barrie Mecrow and Dr. Shady Gadoue

Electrical Power Research Group

School of Electrical and Electronic Engineering

Newcastle University

• Background

• Conventional SRM Control Method

• Direct Torque Control Method for a 6-phase SRM

• Simulation

• Conclusion

Outline

Rotor: no winding or permanent magnet. Stator: concentric windings. Advantages: Simplest structure of all electrical motors with high fault-tolerant. rotor of a six phase SRM stator of a six phase SRM

Background

Applications: aerospace, electric vehicles, high-speed drives, small automotive applications, cooling fans and pumps

Background

relationship between rotor position, current and torque

relationship between rotor position, flux and current

Disadvantages:

Doubly salient structure

Highly unlinear magnetization characteristics

Ununiform torque output

Large torque ripple:

vibration of stator

main source of noise

application limited

0 0.005 0.01 0.015 0.028

9

10

11

12

13

14

15

16

Time(s)

Torque Output(Nm)

30%

Conventional SRM Control Method

Torque command

Current or voltage

reference profile

Torque sharing function

(TSF)

0

10

20

0 0.005 0.01 0.015 0.02 0.025 0.03 0.035 0.04 0.045 0.050

5

10

15

Time(s)

12 A 14 A18 A16 A

10 A

Torque output of single phase()Nm

Current of single phase(A)

-500

0

500

Voltage of single phase/(V)

0 0.005 0.01 0.015 0.02 0.0250

10

20

30

Time(s)

Torque output of single phase(Nm)

Single phase torque output driven by Current Chopping Control

Single phase torque output driven by Angle Position Control

-500

0

500

Voltage of single phase/(V)

0 0.005 0.01 0.015 0.02 0.0250

10

20

30

Time(s)

Torque output of single phase(Nm)0

10

20

0 0.005 0.01 0.015 0.02 0.025 0.03 0.035 0.04 0.045 0.050

5

10

15

Time(s)

12 A 14 A18 A16 A

10 A

Torque output of single phase()Nm

Current of single phase(A)

Q1: If we can control instantaneous torque directly,

rather than by controlling current or voltage,

can we achieve a better torque performance?

Q2: How can we control the torque directly?

Current Torque

Conventional SRM Control Method

Torque equation of SRMs on steady sate

>0 , T>0, flux acceleration

Basic Principle of DTC in SRMs

<0 , T<0, flux deceleration

Basic principle of DTC in SRMs

(a) Stator flux linkage vector of the motor is kept at a constant by selecting an

suitable voltage vector.

(b) Torque can be controlled by accelerating or decelerating the stator flux vector

relative to the rotor movement.

Current in SRM is unipolar, i is positive.

flux linkage discrete expression

the change of stator flux linkage is

be controlled by selecting an

suitable voltage vector

N2

N1

N3 N4

N5

N6

N7

N8

N9 N10

N12

N11

DTC Method for a 6-phase SRMs

U1

U2(ϕb)

U3

U4(ϕc)

U5

U6(ϕd)

U7

U12(ϕa)

U11

U10(ϕf)

U9

U8(ϕe)

x

y

Space Voltage Vectors

U1 (+1,+1,-1,-1,-1,-1)

U3 (-1,+1,+1,-1,-1,-1)

U5 (-1,-1,+1,+1,-1,-1)

U7 (-1,-1,-1,+1,+1,-1)

U9 (-1,-1,-1,-1,+1,+1)

U11 (+1,-1,-1,-1,-1,+1)

U2 (+1,+1,+1,-1,-1,-1)

U4 (-1,+1,+1,+1,-1,-1)

U6 (-1,-1,+1,+1,+1,-1)

U8 (-1,-1,-1,+1,+1,+1)

U10 (+1,-1,-1,-1,+1,+1)

U12 (+1,+1,-1,-1,-1,+1)

Flux and voltage vector arrangement

ϕs Asymmetric Half Bridge

Converter (AHB)

“＋1” = ＋Vdc

“－1” =－Vdc

Zone1~Zone12 are defined to locate stator flux

DTC Method for a 6-phase SRMs

N2

N1

N3 N4

N5

N6

N7

N8

N9 N10

N12

N11

U1

U2(ϕb)

U3

U4(ϕc)

U5

U6(ϕd)

U7

U12(ϕa)

U11

U10(ϕf)

U9

U8(ϕe)

ϕs

Operation example

Assume:

1. Stator flux rotates along anticlockwise and

locates in Zone1.

2. Both flux and torque feedbacks are lower than

reference value.

How can we choose voltage vectors?

Step1: Select voltage vector to increase stator flux

U11, U12, U1, U2, U3

Step2: Select Voltage vectors to increase torque

U2, U3

ϕs

Current Chopping Control(CCC)

N=1000rpm Vdc=560V

Torque reference=13 N·m

Current reference=12A

Direct Torque Control(DTC)

N=1000rpm Vdc=560V

Torque reference=13 N·m

Flux reference=0.52 Wb

Simulation

Current

Hysteresis

Controller

I_ref AHB

Converter M

I_real

＋

－

Torque

Hysteresis

Controller T_ref

AHB

Converter M

T_real

＋

－

Flux

Hysteresis

Controller

Flux_ref Voltage

vector

controller

Flux_real

＋

－

Control diagram of current chopping control

Control diagram of direct torque control

Flux Position

Simulation

Current(A)

Flux Linkage(Wb)

Torque(N·m)

0

0.2

0.4

0.6

0

5

10

15

0 0.005 0.01 0.015 0.02

0

2

4

6

8

Time(s)

0

0.1

0.2

0.3

0.4

0

5

10

15

0 0.005 0.01 0.015 0.02

0

5

10

15

Current(A)

Flux Linkage(Wb)

Torque(N·m)

Time(s)

Current Chopping Control Direct Torque Control

Simulation

0 0.005 0.01 0.015 0.020

2

4

6

8

10

12

14

16

Time(s)

Current Chopping Control

Direct Torque Control

33.3% 11.5%

Stator flux linkage vector comparison Torque output comparison

X-Y stationary frame

Highly linear trajectory

Relatively rounded

-1 -0.5 0 0.5 1-1

-0.5

0

0.5

1

X Axis(Wb)

Y A

xis

(Wb)

X Y Plot

Direct Torque Control

Current Chopping Control

Same average torque output =13.1Nm

Torque ripple sharply reduced

DTC method has been used on a 6-phase SRM

By using DTC method, producing a same average torque, the torque ripple of

DTC method is sharply reduced.

The challenges of DTC method is to select reasonable voltage vectors and

chose flux and torque reference value properly to ensure the efficiency.

Conclusion